(1) Field of the Invention
The present invention relates to an exhaust emission control system for a multiple cylinder internal combustion engine. In particular, the present invention relates to the technique for determining whether a catalytic converter has deteriorated or not.
(2) Description of the Related Art
Conventionally, considering that the oxygen storage capacity of a catalyst in a catalytic converter for exhaust emission control is correlated closely with the catalyst performance (mainly the HC purifying performance), whether the catalytic converter containing a large amount of oxygen occluding substances such as cerium (Ce) has deteriorated or not is determined by monitoring a change in the oxygen storage performance with passage of time.
This catalyst deterioration detection method utilizes such a characteristic that, in the case where the air-fuel ratio of an exhaust gas flowing into the catalytic converter is modulated between a lean air-fuel ratio and a rich air-fuel ratio at a predetermined cycle with a predetermined amplitude, if the oxygen storage capacity is high, the response or amplitude of the air-fuel ratio of the exhaust gas downstream of the catalyst is relatively slow or small since the oxygen is occluded by the catalytic converter, and if the oxygen storage capacity is low, the response or amplitude of the air-fuel ratio of the exhaust gas downstream of the catalyst is relatively quick or large since the oxygen is emitted without being not so much occluded by the catalytic converter. For example, the variation cycle, amplitude, or the like of the oxygen concentration detected and outputted from an oxygen sensor (O2 sensor) or an air-fuel ratio sensor (LAFS) provided downstream of the catalyst is detected, and if the detected variation cycle, amplitude, or the like is equal to or greater than a predetermined reference value, it is then determined that the oxygen storage capacity has lowered, i.e. the catalytic converter has deteriorated.
Incidentally, in recent years, the exhaust emission control performance has been desired to be further improved in view of environmental protection, and accordingly, even a slight deterioration of a catalytic converter has been required to be detected.
According to the above-described conventional catalyst deterioration detecting method, however, the modulation cycle is set to a relatively short cycle due to the relation with the operational performance of the internal combustion engine. Since only a small amount of oxygen can be occluded on such a short modulation cycle and there is only a slight variation in the output value of the oxygen concentration, it is difficult to detect a slight deterioration of the catalyst. Namely, although it is possible to detect a deterioration of the catalyst to a certain extent, it is difficult to detect a slight deterioration of the catalyst.
To address this problem, it may be possible to consider increasing the modulation cycle. The increase in the modulation cycle causes oxygen to be occluded to such an extent as to exceed the oxygen storage capacity, and it is therefore possible to surely detect a change with elapse of time in modulation cycle, amplitude, or the like of the output value of the oxygen concentration and to detect even a slight deterioration of the catalyst. In this case, however, there is the problem that the NOx purifying performance is deteriorated.
FIG. 5 is a graph showing the relationship between the NOx emission and the modulation frequency in the case where a middle air-fuel ratio is changed when the exhaust air-fuel ratio is modulated. In FIG. 5, “o” indicates the shortest cycle (highest frequency), and the frequency is increased in the order of “□,” “Δ,” and “▴,” and “▪” indicates the longest cycle (the lowest frequency). When the cycle is increased (the frequency is lowered), the NOx emission is reduced if the middle air-fuel ratio is modulated to a leaner air-fuel ratio, but the NOx emission is increased if the middle air-fuel ratio is modulated to a richer air-fuel ratio.
Further, although not shown in FIG. 5, increasing the cycle (lowering the frequency) may cause an increase in the NOx emission irrespective of the middle air-fuel ratio.
It is therefore an object of the present invention to provide an exhaust emission control system for an internal combustion engine capable of surely determining whether or not catalytic converters have deteriorated even slightly while ensuring a high exhaust emission control performance.